The aim of this research is to elucidate the control of differentiation of a multipotent stem cell, the interstitial cell (i-cell of hydra. I (with others) have developed a powerful method for examining i-cell differentiation, which coupled with the simplicity of hydra allows me to explore this problem in a manner not available in other systems. The method involves dissociating hydra into single cells and forming cell aggregates which will develop into normal animals. By separating the cell types, animals of selected cell compositions can be constructed. With this method, and by the selective deletion of a cell type(s) in intact animals, i-cells will be placed in specific cellular environments and their patterns of differentiation studied. These patterns will be analyzed quantitatively as methods have been developed for quantitatively measuring all cell populations in hydra, as well as quantitatively examining isotopically-labelled cohorts of cells with autoradiography. Isotopically labelled i-cells can be identified even after five cell divisions. I will study i-cell differentiations into four types of nematocytes because: (1) each has highly specific morphological characteristics; (2) each of the four populations has been thoroughly studied; (3) i-cell yield nematocyte differentiation is regulated by stimuli external to the i-cells; (4) i-cell yields nematocyte differentiation depends in part on position in the body column. The objects of the proposal are to: (1) study the nature of the controls on i-cell population size and their effect on i-cell differentiation; (2) study the influence of nematocytes and nerve cells on i-cell yields nematocyte differentiation; (3) study the basis of position-dependent i-cell differentiation; (4) study the reversibility of i-cell commitment.